In eukaryotic cells DNA is packaged with histones, to form chromatin. Approximately 150 base pairs of DNA are wrapped twice around an octamer of histones (two each of histones 2A, 2B, 3 and 4) to form a nucleosome, the basic unit of chromatin. The ordered structure of chromatin needs to be modified in order to allow transcription of the associated genes. Transcriptional regulation is key to differentiation, proliferation and apoptosis, and is, therefore, tightly controlled. Control of the changes in chromatin structure (and hence of transcription) is mediated by covalent modifications to histones, most notably of the N-terminal tails. Covalent modifications (for example methylation, acetylation, phosphorylation and ubiquitination) of the side chains of amino acids are enzymatically mediated (A review of the covalent modifications of histones and their role in transcriptional regulation can be found in Berger SL 2001 Oncogene 20, 3007-3013; See Grunstein, M 1997 Nature 389, 349-352; Wolffe A P 1996 Science 272, 371-372; and Wade P A et al 1997 Trends Biochem Sci 22, 128-132 for reviews of histone acetylation and transcription).
The Aurora kinases are a family of serine/threonine kinases which have been identified as key regulators of the mitotic cell division process (Bischoff and Plowman, 1999 Trends Cell Biol 9, 454-459) which may become deregulated in cancer and other hyperproliferative diseases (Warner et al, 2003, Mol Can Ther 2, 589-595). The three members of this family identified so far are referred to as Aurora-A, Aurora-B and Aurora-C. Higher eukaryotic cells typically express two or more Aurora kinases. It has been shown that inhibition of Aurora B affects several facets of mitosis including histone H3 phosphorylation, chromosome segregation and cytokinesis. Aurora A and C localise to spindle poles with Aurora A being required for bipolar spindle formation in a number of systems (Giet and Prigent, 1999, J. Cell. Sci 11, 3591-3601). They have been identified as homologues of Ip11, a prototypic yeast kinase and the Drosophila aurora kinases. Aurora A and B have been shown to be overexpressed in a number of human cancers and their overexpression in cells in vitro leads to transformation, centrosome abnormalities and aneuploidy (Bischoff et al, 1998, EMBO J. 17, 3052). Cells which overexpress Aurora A have been shown to form tumours in aythymic mice. The observations contained in these manuscripts suggest that increase in Aurora kinase activity may serve to promote tumour development by providing growth advantage or by inducing genetic instability and that Aurora Kinase inhibition should have therapeutic benefit in cancer, and other proliferative diseases.
Aurora Kinase Inhibitors.
The following patent publications relate to Aurora kinase inhibitors and their preparation: WO 02/00649, WO 2004/000833, WO 03/055491, WO 2004/058752, WO 2004/058781, U.S. Pat. No. 6,143,764 and US 2004/0049032. Many of the known inhibitors are quinolines and quinazolines which conform to the general structural template:
wherein Q is ═CH—, ═C(CN), ═C(Br), ═C(cyclopropyl) or ═N—, the group Ra is variable but often a small alkoxy group such as methoxy, the group Rb is a solubilising group, W is a hetero radical such as NH or O, A is an aromatic or heteroaromatic ring, L1 is a linker radical, usually containing nitrogen and carbonyl, and ring B is an optional (r=0 or 1) aromatic or heteroaromatic ring. The —W-A-L1-(B)r—H can be thought of as the side chain of the quinoline/quinazoline ring system, and it is the quinoline/quinazoline plus side chain which plays the major role in binding to the Aurora kinase enzyme. The substituent Rb appears to be oriented away from the bound enzyme, and is therefore a suitable location for modification to improve properties such as solubility.
The present invention is based on the finding that certain novel modifications of the substituent in the Rb position (referred to above) of quinoline- and quinazoline-type Aurora kinase inhibitors lead to desirable pharmacokinetic improvements relative to known inhibitors. In particular, it has been found that incorporating an alpha amino acid ester moiety in that substituent facilitates transport into the cell, where the Aurora kinase is of course located. There, the ester is cleaved by intracellular esterases, releasing the parent acid, which is not readily transported out of the cell. The accumulation of the ester and its esterase hydrolysis product within the cell results in concentration of Aurora kinase inhibitory activity where it is needed.